U.S. patent number 4,736,284 [Application Number 06/947,505] was granted by the patent office on 1988-04-05 for switching power supply circuit including forward converter.
This patent grant is currently assigned to Kikusui Electronics Corp.. Invention is credited to Masahiko Shimizu, Masaaki Yamagishi.
United States Patent |
4,736,284 |
Yamagishi , et al. |
April 5, 1988 |
Switching power supply circuit including forward converter
Abstract
In a switching power supply circuit, a forward converter
includes first to third switching transistors and a switching
transformer having first and second primary windings. The first to
third switching transistors are connected via series-connected
first and second primary windings to a high voltage DC source.
While the first to third transistors are simultaneously turned off,
magnetic fluxes induced in the switching transformer are completely
reset.
Inventors: |
Yamagishi; Masaaki (Tokyo,
JP), Shimizu; Masahiko (Yokohama, JP) |
Assignee: |
Kikusui Electronics Corp.
(Kawasaki, JP)
|
Family
ID: |
11615345 |
Appl.
No.: |
06/947,505 |
Filed: |
December 29, 1986 |
Foreign Application Priority Data
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Jan 14, 1986 [JP] |
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61-5586 |
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Current U.S.
Class: |
363/16; 363/20;
363/24; 363/97 |
Current CPC
Class: |
H02M
3/3353 (20130101) |
Current International
Class: |
H02M
3/335 (20060101); H02M 3/24 (20060101); H02M
003/335 () |
Field of
Search: |
;363/24,25,26,16-21,56,97,133,134 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3133578 |
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Mar 1983 |
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DE |
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61-66562 |
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May 1986 |
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JP |
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Other References
The Switchmode Guide (Motorola), R. J. Haver, "The Designer's Guide
for Switching Power Supply Circuits and Components", Motorola
Semiconductor Products Inc..
|
Primary Examiner: Salce; Patrick R.
Assistant Examiner: Peckman; Kristine
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Claims
What is claimed is:
1. A switching power supply circuit comprising:
forward converter means coupled to a DC (direct current) power
supply means having positive and negative terminals;
said forward converter means including:
(1) switching transformer means having first and second primary
windings and a secondary winding magnetically coupled to said first
and second primary windings, said first and second primary windings
being coupled in series with said positive and negative terminals
of the DC power supply means;
(2) first switching means connected between said positive terminal
and one end of the first primary winding;
(3) second switching means connected between the other end of the
first primary winding and one end of the second primary
winding;
(4) third switching means connected between the other end of the
second primary winding and the negative terminal, said first,
second and third switching means being series-coupled via said
series-connected first and second primary windings to the DC power
supply means, and arranged to be substantially simultaneously
turned on/off; and
(5) means coupled between each end of said first and second primary
windings and the DC power supply means, for returning to the DC
power supply means a counterelectromotive force induced in said
first and second primary windings, while said first to third
switching means are substantially simultaneously turned off,
wherein electromagnetic fluxes induced in the switching transformer
means are reset during a turning-off period of said first to third
switching means.
2. A circuit as claimed in claim 1, wherein said
counterelectromotive-force-returning means are diode means.
3. A circuit as claimed in claim 2, wherein said diode means are
Schottky diodes.
4. A circuit as claimed in claim 1, wherein said first and second
primary windings of the switching transformer means have the same
turn numbers.
5. A circuit as claimed in claim 1, wherein said first to third
switching means are bipolar transistors.
6. A circuit as claimed in claim 5, wherein said bipolar
transistors have a withstanding voltage of approximately 450 V and
a switching frequency of approximately 100 KHz.
7. A circuit as claimed in claim 1, wherein said first to third
switching means are field-effect transistors.
8. A circuit as claimed in claim 7, wherein said field-effect
transistors have a withstanding voltage of approximately 450 V and
a switching frequency of approximately 100 KHz.
9. A circuit as claimed in claim 1, wherein said forward converter
means is connected to a DC power supply capable of applying DC
voltages approximately from 200 to 370 volts.
10. A circuit as claimed in claim 1, further comprising:
rectifier and filtering means connected to said secondary winding
of the switching transformer means, for rectifying and filtering AC
(alternating current) output from the secondary winding thereof to
derive a DC output voltage, wherein said forward converter means is
arranged to operate as a DC-to-DC forward converter.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a switching power supply
circuit, and more particularly to a high-frequency converter
arranged to operate at a high duty ratio.
2. Description of the Prior Art
Various types of switching power supply circuits have been
developed as, for instance, forward converters, push-pull
converters and bridge converters.
As is well known in the art, forward converters have particular
advantages in that no asymmetrical flux swings, or dissymmetry
phenomenon occurs, as compared with push-pull converters and bridge
converters. In push-pull converters and bridge converters,
generally, the asymmetrical flux swing phenomenon occurs during
transition periods, in load fluctuations or in aging effects of
switching transistors, because excited magnetic fluxs cannot be
reset during turn-off periods of switching transistors.
FIG. 1 shows a typical double ended DC-to-DC forward converter.
A basic operation of the forward converter is as follows. An
electric power is transferred from a primary circuit of a switching
transformer to a secondary circuit thereof during turn-on periods
of switching transistors. These switching transistors are simply
represented by switch contacts. An exciting energy of the switching
transformer is reset during turn-off periods of the switching
transistors.
The magnetic excitation in the switching transformer will be simply
explained. When a voltage E.sub.i of a DC power supply 11 is
applied to the switching transformer having the primary winding of
a turn number n.sub.1 for only a time period T.sub.on, a magnetic
flux .phi..sub.set generated in the core of the switching
transformer is given by the following equation (1). ##EQU1##
That is, the magnetic flux .phi..sub.set is given by the product of
applied voltage E.sub.i and period "T.sub.on " for applying this
voltage and is equivalent to an electric power which passes through
the switching transformer. In other words, a total amount of
magnetic flux .phi..sub.set is not influenced by the electric power
which is transferred to the secondary winding.
A typical magnetic material has a limit value which is defined by
the maximum magnetic flux density. This flux density is equal to a
value which is obtained by dividing the produced magnetic flux by
the effective sectional area of the core, and is namely, the
magnetic flux per unit area. If the magnetic material is excited to
a value in excess of the limit value, it will be saturated and
therefore the permeability of the magnetic material will promptly
decrease. Thus, the inductance of the primary winding is rapidly
approximated to the inductance of the air-core coil, so that the
switching transformer utilizing such a magnetic material will fail
to perform its transformer function.
Therefore, the magnetic flux excited during the preceding turn-on
(T.sub.on) period needs to be firmly reset during the turn-off
(T.sub.off) period of a switching transistor.
For a better understanding of the foregoing fundamental operation,
the operation of the double ended type DC-to-DC forward converter
in FIG. 1 will now be explained hereinbelow.
The conventional DC-to-DC forward converter includes first and
second switching transistors 1A and 1B, a switching transformer 8
having a primary winding 3 and a secondary winding 5; and first and
second feedback diodes 2A and 2B. The first and second switching
transistors 1A and 1B are series-connected to the primary winding 3
of the switching transformer 8. The first and second feedback
diodes 2A and 2B are connected between the DC power supply 11 and
the corresponding switching transistors 1A and 1B in such a manner
that the reverse current can be fed back, or returned to DC power
supply 11 namely to the forward converter during the turn-off
periods of the switching transistors 1A and 1B. To the secondary
winding 5, a rectifier and smoothing filter circuit having a
rectifier diode 4 is connected to derive a DC output. The turn
number of the primary winding 3 is selected to be n1, whereas that
of the secondary winding 5 is n2.
The fundamental operation of the forward converter in the double
ended DC-to-DC forward converter having the above circuit
arrangement will now be explained.
In the converter in FIG. 1, when both switching transistors 1A and
1B are turned off, the counter electromotive force which has been
generated in primary winding 3 of switching transformer 8, is
returned to DC power supply E.sub.i diodes 2A and 2B, so that the
voltage across primary winding 3 is clamped to the power source
voltage and the excited magnetic flux is reset.
A magnetic flux .phi..sub.res to be reset is given by the following
equation (2). ##EQU2##
Therefore, the condition regarding the magnetic excitation
necessary to make the converter in FIG. 1 operative will be given
by:
By substituting the equations (1) and (2) for the inequality (3),
we have ##EQU3##
The theoretical limit value of the duty ratio ##EQU4## since
T.sub.on .ltoreq.T.sub.off. However, the theoretical limit value is
practically reduced to a value of up to 30 to 40% due to safety
reasons.
The single transistor converter shown in FIG. 2 is of the type in
which a switching transformer 9 has a third winding 6 for resetting
the magnetic excitation. In the above prior art converter, the
magnetic flux excited is returned to DC power source E.sub.i
through a diode 2C.
Magnetic flux .phi..sub.res to be reset in this case is given by
the following equation (5). ##EQU5## As can be understood from this
equation, when a turn number n.sub.3 of third winding 6 is reduced,
the magnetic flux can be sufficiently reset even if the turn-off
period T.sub.off is reduced.
As an example, assuming that a turn ratio is set to n.sub.3
=0.5n.sub.1, the limit value of the duty ratio will amount to
approximately 67%.
However, since primary and third windings 3 and 6 are magnetically
coupled to each other in switching transformer 9, when diode 2C is
turned on and the voltage across third winding 6 is clamped to the
power source voltage of -E.sub.i while switching transistor 1 is
turned off, a great counter electromotive force given below is
generated in primary winding 3. ##EQU6## This counter electromotive
force is superimposed on voltage E.sub.i of DC power supply 11 and
applied to a switching transistor 1, so that the total voltage
inevitably becomes: ##EQU7##
However, as described above, the conventional forward converters in
FIGS. 1 and 2 have an advantage such that the excited energy can be
reset during the turn-off period of the switching transistor. In
other words, the forward converter has an advantage such that there
is no saturation of the core of the switching transformer by an
asymmetrical flux swing which causes a problem in the conventional
push-pull type or bridge type converter. Nevertheless, there are
drawbacks such that the duty ratio cannot exceed 50% and the
efficiency of the switching transformer cannot be effectively
increased.
In addition, such a forward converter is generally operated at a
high input voltage, e.g., at a DC voltage of 200 to 370 V and
further performs the switching operation at a high frequency on the
order of, e.g., 100 kHz. Therefore, if the forward converters as
shown in FIGS. 1 and 2 are made operative by use of commercially
available switching transistors, the duty ratio cannot be
sufficiently great, and there is the risk such that these
transistors are broken down by the counter electromotive voltage,
which is about three times as high as the power source voltage as
mentioned above.
In addition, there is also another problem such that if the
commercially available switching transistors of reasonable prices
are employed, the design of the circuit elements and operating
conditions will be limited, due to the foregoing problems.
It is, therefore, an object of the present invention to provide a
switching power supply circuit in which the duty ratio can exceed
50%, the asymmetrical flux swing (DC excitation) phenomenon does
not occur, and the magnetic flux excited during the switching-on
periods can be sufficiently reset.
It is another object of the invention to provide a switching power
supply circuit which can perform the switching operation at a high
source voltage and at a high frequency.
SUMMARY OF THE INVENTION
These objects of the invention are accomplished by providing a
switching power supply circuit comprising:
forward converter means coupled to a DC (direct current) power
supply means having positive and negative terminals;
said forward converter means including:
(1) switching transformer means having first and second primary
windings and a secondary winding magnetically coupled to the first
and second primary windings, the first and second primary windings
being coupled in series with the positive and negative terminals of
the DC power supply means;
(2) first switching means connected between the positive terminal
and one end of the first primary winding;
(3) second switching means connected between the other end of the
first primary winding and one end of the second primary
winding;
(4) third switching means connected between the other end of the
second primary winding and the negative terminal, the first, second
and third switching means being series-coupled via said
series-connected first and second primary windings to the DC power
supply means, and being substantially simultaneously turned on/off;
and
(5) means coupled between each end of the first and second primary
windings and the DC power supply means, for returning to the DC
power supply means a counter-electromotive force induced in said
first and second primary windings, while said first to third
switching means are substantially simultaneously turned off,
whereby electromagnetic fluxes induced in the switching transformer
means are reset during a turning-off period of said first to third
switching means.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of these objects and other objects of
the present invention, reference is made to the following detailed
description of the invention to be read in conjunction with the
following drawings, in which:
FIG. 1 is a circuit diagram of a conventional double ended DC-to-DC
forward converter;
FIG. 2 is a circuit diagram of a prior art single transistor
DC-to-DC forward transistor.
FIG. 3 is a circuit diagram of a DC-to-AC forward converter
according to a first preferred embodiment of the invention;
FIG. 4A shows waveforms of secondary winding voltage and
transformer output voltage derived from the conventional forward
converter shown in FIG. 1;
FIG. 4B illustrates waveforms of secondary winding voltage and
transformer output voltage derived from the forward converter shown
in FIG. 3;
FIG. 5 is a circuit diagram of a DC-to-AC forward converter
according to a second preferred embodiment of the invention;
and,
FIG. 6 is a circuit diagram of a practical DC-to-DC forward
converter according to a third preferred embodiment of the
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
CIRCUIT ARRANGEMENT OF FIRST FORWARD CONVERTER
Referring now to FIG. 3, a first DC-to-AC forward converter
according to the invention will be described. The first forward
converter includes a series connection between both ends (positive
and negative terminals) of DC power supply 11. This series
connection is constructed of a first switching element 12; a first
primary winding 22A of a switching transformer 20; a second
switching element 14; a second primary winding 22B of switching
transformer 20; and a third switching element 15.
Switching elements 12, 14, and 15 may be, e.g., switching
transistors and are simultaneously turned on or off by a control
circuit (not shown in detail).
Switching transformer 20 has two sets of primary windings 22A and
22B having turn numbers "np.sub.1 " and "np.sub.2 " which are
electrically equal to each other, and a secondary winding 22C
having the turn number "np.sub.3 ".
A series connection point of second switching element 14 and first
primary winding 22A and series connection point of third switching
element 15 and second primary winding 22B are connected to a
positive polarity terminal PT of DC power source 11 through
feedback diodes 16 and 17 in the forward directions,
respectively.
On the other hand, a series connection point of first switching
element 12 and first primary winding 22A and a series connection
point of second switching element 14 and second primary winding 22B
are connected to a negative polarity terminal NT of DC power source
11 through feedback diodes 18 and 19 in the opposite directions,
respectively.
SWITCHING OPERATION
In the power supply circuit in FIG. 3, when switching elements 12,
14, and 15 are substantially simultaneously turned on, voltage
E.sub.i of DC power supply 11 is applied to first and second
primary windings 22A and 22B of switching transformer 20.
Magnetic flux .phi..sub.set which is accumulated in the core of
switching transformer 20 during on-time period T.sub.on is given by
the following equation (8) similarly to the equation (1) when the
turn-on time of the switching transistor assumes "T.sub.on ".
##EQU8##
When switching elements 12, 14, and 15 are turned off, the current
flowing through primary windings 22A and 22B cannot be suddenly
turned off because of the inductance of the primary winding itself,
as is well known, but is intended to continuously flow.
That is, the counter electromotive forces occur in primary windings
22A and 22B. The counter electromotive force generated in first
primary winding 22A forwardly biases feedback diodes 16 and 18, and
the voltage across first primary winding 22A is clamped by voltage
E.sub.i of DC power supply 11 and then, the primary current is
returned to power supply 11.
Similarly, the counter electromotive force generated in second
primary winding 22B forwardly biases diodes 17 and 19, and the
voltage across second primary winding 22B is clamped by voltage
E.sub.i of DC power supply 11 and then, the primary current is
returned to power supply 11.
As a result, the voltage which is applied while switching elements
12, 14, and 15 are in the OFF state does not exceed voltage E.sub.i
of power supply 11.
On the other hand, magnetic flux .phi..sub.res which is reset while
the switching elements are in the OFF state is given by:
##EQU9##
Equation (9) is based on the fact that when switching elements 12,
14, 15 are turned off, a fly-back voltage is produced, and primary
windings 22A and 22B are clamped with the same voltage E.sub.i.
That is, a reverse current flows from primary windings 22A, 22B to
power source 11, since primary windings 22A and 22B form an
equivalent parallel circuit. At this time, the fluxes passing
through the core of the switching transformer reduce. The reduction
of fluxes is described as "reset", and the following Faraday law is
established: ##EQU10##
The reset does not occur by forcefully applying a voltage from an
external circuit, but by emitting an energy due to fly-back voltage
generated from the transformer. Such resetting can be realized in
the secondary side as well as in the primary side. ##EQU11## Es:
voltage produced in secondary winding Ei: voltage produced in
primary winding
ns: number-of-turns of secondary winding
np1: number-of-turns of first primary winding
np2: number-of-turns of second primary winding.
In the present embodiment, a clamp circuit for a fly-back
transformer is provided on the primary side.
Where np1=np2, the exciting condition necessary to make the
DC-to-AC converter shown in FIG. 3 normally operative is shown by
the equation (3).
By substituting the equations (8) and (9) for the inequality (3)
and by setting np.sub.1 =np.sub.2, ##EQU12## next, by substituting
the condition obtained from the equation (11), i.e., (1/2)T.sub.on
=T.sub.off for the equation of the duty ratio as previously
mentioned, we have ##EQU13## Therefore, the limit value of the duty
ratio is about 66.7%. As compared with the duty ratios of the
converters illustrated in FIGS. 1 and 2, the duty ratio of this
first forward converter can be remarkably reduced for the turn-off
period. In other words, according to the present embodiment, there
is an important feature such that magnetic flux .phi..sub.res which
can be completely reset while switching elements 12, 14, and 15 are
in the off state can be remarkably reset within a short time (30%
versus 67%).
As mentioned, above, there is another feature such that the counter
electromotive voltage in the off state can be suppressed to a low
voltage, i.e., power source voltage E.sub.i.
According to the present embodiment, the turn-on periods of
switching elements 12, 14, and 15 can be set to the long periods.
Therefore, there is still another feature such that the power
transmission efficiency from the primary circuit to the secondary
circuit of switching transformer 20 is remarkably improved.
Therefore, in the case of producing the same output voltage E.sub.0
by the forward converter as that by the conventional forward
converter, illustrated in waveforms of FIG. 4, the voltage of the
secondary winding becomes 3.33 E.sub.0 in the conventional forward
converter which operates the duty ratio of 30% (FIG. 1). As
compared with this conventional example, in the forward converter
of the preferred embodiment of FIG. 3 having the duty ratio of,
e.g., 60%, it is sufficient to design the voltage of the secondary
winding to 1.66 E.sub.0.
Thus, the turn number of secondary winding 22C can be reduced to
about half as compared with the conventional one. The inductance of
a smoothing choke coil, which will be explained hereinafter, in the
output circuit of the converter can be also reduced by one
half.
Further, the circuit arrangement of the present invention can be
accomplished by merely adding, for example, one switching
transistor and two feedback diodes to the conventional forward
converter and the simple circuit arrangement with high power
efficiency is also realized. The forward converter in the present
embodiment can be stably operated at the duty ratio of 60% without
generating any DC exciting flux. For example, if there is no change
in the electric power capacity of the materials of the switching
elements, switching transformer, and the like, the electric power
which is about 1.5 times larger than that of the conventional
forward converter having the duty ratio of 40% can be supplied to
the load.
CIRCUIT ARRANGEMENT OF SECOND FORWARD CONVERTER
Referring now to FIG. 5, a second DC-to-AC forward converter will
be described. As is easily understood from the circuit of this
second forward converter, all of the circuit elements are identical
to those employed in the circuit of the first forward converter
shown in FIG. 3. The cathode of feedback diode 17 of is, however,
to the anode of feedback diode 16, whereas the anode of feedback 18
is connected to the cathode of feedback diode 19. The remaining
circuit portion of the second forward converter is identical to
that of the first forward converter. Thus, the second forward
converter can be operated in a similar manner to the first forward
converter.
As described above, according to the invention, the duty ratio can
be increased without the core saturation. Thus, it is possible to
provide a switching power supply circuit which can improve the
power efficiency of the switching transformer, can reduce the
withstanding voltage of the switching element, and can supply a
large electric power to a load.
Further, according to the invention, it is possible to provide a
switching power supply circuit which can perform the switching
operation at an input voltage of a high main voltage of, e.g., 200
to 240 V and at a high frequency of, e.g., 100 kHz.
In addition, according to the invention, the counter electromotive
voltage which is generated during the turn-off period of the
swiching elements can be considerably reduced. Thus, there is an
advantage such that cheaper switching elements can be used.
PRACTICAL CIRCUIT DESIGN
FIG. 6 shows a practical circuit of a DC-to-DC forward converter
according to the present invention.
As will be obvious from this circuit diagram, a filtering circuit
connected to secondary winding 22C of switching transformer 20 is
coupled with the DC-to-AC forward converter of FIG. 3. This
filtering circuit comprises rectifier diodes 30 and 32; a choke
coil 34; and a filtering capacitor 36. Such a rectifier and
smoothing circuit itself is well known.
The third forward converter according to the invention employs the
following circuit components.
TABLE ______________________________________ Input voltage (AC)
200-240 V .+-. 10% Input voltage (DC) 200-370 V switching FETs 12,
14, 15 2SK 559 (available from Hitachi) 450 V 15 A switching
transformer 20 PQ 35/35 H7Cl (ferrite core of TDK) primary windings
np.sub.1, np.sub.2 :22 turns secondary winding np.sub.3 :3 turns
feedback diodes 16 to 19 RG 4, 400 V 2.4 A (available from
SHINDENGEN K.K.) rectifier diodes 30, 32 C80 HO 40 40 V 80 A
(available from NIHON INTERNATIONAL RECTIFIER K.K.) Choke coil 34 6
.mu.H 60 A filtering capacitor 36 10 V 10,000 .mu.F .times. 6 pcs.
switching frequency approx. 100 KHz
______________________________________
The DC-to-DC forward converter employing such components has a
feature such that it is operative at a high input voltage of 200 to
370 DCV and at a high frequency of the order of 100 kHz.
In this case, there is an advantage such that the components used
in this DC-to-DC forward converter are all inexpensive and easily
commercially available.
While the present invention has been described in terms of certain
preferred embodiments, and exemplified with respect thereto, those
skilled in the art will readily appreciate that various
modifications, changes, omissions, and substitutions may be made
without departing from the spirit and scope of the invention.
For example, as switching elements, not only bipolar transistors
but also FETs can be used. On the other hand, any type of diodes
can also as used as the feedback and rectifier diodes if they have
good high frequency characteristics. For example, Schottky diodes
can be also used.
* * * * *